The present invention relates to a method and an apparatus for evaluating a dielectric film. In particular, the present invention relates to lifetime prediction of dielectric breakdown.
In accordance with recent improvement in the packing density of semiconductor integrated circuit devices, the feature sizes of device elements have kept on being shrunk. In the field of VLSIs, the thickness of a silicon dioxide (SiO.sub.2) film used as a gate oxide has become smaller than 10 nm, and lifetime prediction of such a thin oxide film has become more and more important. As an evaluating method for gate oxides, a constant voltage stressing method or a constant current stressing method is widely used.
With reference to FIG. 18, a conventional constant voltage stressing method will be first described.
First, a stressing voltage V.sub.0, a judgement current level I.sub.0 and the number N of samples subjected to measurement are set in step S50.
Next, in step S51, a measurement probe is moved to a first sample selected among the plural samples. Then, in step S52, the stressing voltage V.sub.0 is applied across a dielectric film of the sample. Subsequently in step S53, the application of the stressing voltage V.sub.0 across the dielectric film is retained for a time period of t1 seconds. A leakage current (I) across the dielectric film is measured in step S54. In step S55, whether dielectric breakdown has occurred is determined on the basis of the measured current level I. For example, it is determined that the dielectric breakdown has occurred when the absolute value of the current level I is larger than the judgement current level I.sub.0. In the case where it is determined that the dielectric breakdown has not occurred, the measurement procedure returns to step S53, so that the steps S53, S54 and S55 are repeated until the dielectric breakdown is observed. In the case where the breakdown was detected in step S55, a time to breakdown is recorded in step S56.
When the measurement for all the samples is completed in step S57, all of the total stressing times(t.sub.1 to t.sub.N) for the N samples are used to calculate a time t.sub.BD in step 58. Weibull plotting can be adopted for such calculation. The Weibull plotting will be described below.
First, values W are calculated on the basis of a cumulative distribution function F and are plotted with regard to each of the stressing times (t.sub.1 to t.sub.N) on a log scale. In general, the value W represents a cumulative percent of failure, and the cumulative distribution function F represents the probability that the device will fail at or before time t. The value W is given by the following expression (1): EQU W=ln [ln {1/(1-F)}] (1)
It is empirically known that a linear relationship can be obtained between the value W and the stressing time t. More specifically, the measurement data plotted in a Weibull plotting paper shows a linear relationship between broken-down oxides and the time t to the breakdown. Fore example, the stressing time t corresponding to W=50% is easily obtained by Weibull plotting. The obtained stressing time t, or t.sub.50, means a time when 50% of the oxides has broken down. The Weibull plotting is widely used to estimate the lifetime of dielectric films.
When the measurement of all the N samples is not completed in step S57, the measurement probe is moved to a subsequent sample (step S59), and the measurement procedure returns to step S52. The steps S52 through S59 are then repeated until the measurement of all the N samples is completed.
The sample number N is generally 20 through 100. This is because the measured time t varies among samples, and hence, the time t.sub.BD cannot be accurately determined if the sample number N is small.
The time t.sub.BD obtained in this manner corresponds to the lifetime of oxide breakdown. Therefore, the time t.sub.BD is used for evaluating the quality and reliability of gate oxides.
Next, with reference to FIG. 19, the conventional constant current stressing method will be described.
First, in step S60, a current level I.sub.0 for stressing, a critical voltage V.sub.0 and the sample numbers N are set and input a measurement apparatus. In step S61, a measurement probe is moved to a first sample.
Next, in step S62, the current I.sub.0 is applied to a dielectric film of the first sample. After t1 seconds from the start of the application of the stressing current I.sub.0 (step S63), a gate voltage V is measured in step S64. In step S65, it is determined whether or not the oxide breakdown has been caused. For example, when the absolute value of the voltage V is smaller than the absolute value of the critical voltage V.sub.0, it is determined that the oxide breakdown has occurred. When it is determined in step S65 that the breakdown has not occurred, the measurement procedure returns to step S62. Then, the steps S63 through S65 are repeated until the oxide breakdown is observed on the first sample.
When the breakdown is detected in step S65, a time t from the start of the current stressing to the oxide breakdown is recorded. When the measurement of all the samples is completed (step S67), the stressing times t with regard to all the samples are used for calculating a lifetime t.sub.BD of these samples and a total injected charge Q.sub.BD (step S68). The time t.sub.BD is determined by using the aforementioned Weibull plotting. Herein, the total injected charge Q.sub.BD is defined as a value obtained by dividing a product of the time t.sub.BD and the stressing current I.sub.0 by an gate electrode area S.
When the measurement of all the samples is not completed in step S67, the measurement probe is moved to a subsequent sample in step S69, so that the procedures in steps S62 through S69 can be repeated until the measurement of all the samples is completed. Also in this case, the sample numbers N is approximately 20 through 100.
In these methods for predicting the lifetime of oxide breakdown, it is disadvantageously necessary to prepare a large number of samples and it takes a disadvantageously long time for the measurement. It is generally known that the measurement error is generally in proportion to (N.sup.1/2)/N. Therefore, when the sample number N is small, the lifetime prediction cannot be reliable. In order to improve the reliability of the lifetime prediction, it is necessary to increase the sample number, which results in an increases in the measurement time.
The object of the present invention is providing a method and an apparatus for evaluating a dielectric film in which the time and the sample number required for measurement can be reduced without degrading the measurement reliability.